Long-distance, high-capacity wavelength division multiplexed transmission systems has become possible owing to improvements of erbium-doped fiber optic amplifiers and transmission fiber. Further, the rapid spread of the Internet has led to increased demand for larger communication capacity in long-distance fiber optic transmission lines, and an increase in such capacity is under consideration.
Generally, in fiber optic transmission, it is known that a large accumulation of wavelength dispersion has the effect of accelerating waveform degradation due to non-linear effects. For this reason, when a transmission line is constructed, care is taken to avoid the occurrence of a large accumulation of dispersion to the greatest extent possible. However, in case of wavelength division multiplexed transmission systems, transmission fiber exhibits wavelength dependence on wavelength dispersion (i.e., a higher-order disoersion characteristic) and, as a consequence, the manner in which dispersion accumulates differs for each channel.
The following discussion will be given by way of analyzing the problems encountered in the prior art in the course of investigation toward the present invention.
An example of a case in which a 16-channel 10-Gb/s signal (0.8 nm spacing) is transmitted 6,000 km will be described. An optical fiber for a transmission line is composed of an NZDSF (Non-zero Dispersion-Shifted Fiber) having a dispersion value on the order of -2.0 [ps/nm/km], and a normal fiber having a dispersion value on the order of +17 [ps/nm/km]. Assume that higher-order dispersion values of these fibers are 0.11 [ps/nm2/km] and 0.06 [ps/nm2/km], respectively.
In order to make the average dispersion value zero, the transmission line is constructed at such a fiber ratio that 2 km of normal fiber is used per 17 km of NZDSF. In such case the average higher-order dispersion value will be (0.11*17+0.06 *2)/(17+2)=0.105 [ps/nm2/km]. In accordance with this result, a difference in accumulated dispersion equal to about 500 [ps/nm] (=0.8*0.105*6,000 km) is produced between neighboring channels, which are separated by 0.8 nm, after transmission over the distance of 6,000 km. The difference will be 8,000 [ps/nm] between the channels at both ends of the 16 channels. The channels at both ends will sustain a dispersion accumulation of .+-.4,000 [ps/nm] even in a case where the design is such that the average dispersion value is zero at the center channel.
Thus, with ultra-long-distance transmission, the manner in which dispersion accumulates differs for each channel because of the higher-order dispersion possessed by transmission fiber. A large dispersion accumulates especially at the end channels. Since this large accumulation of dispersion accelerates non-linear waveform degradation, it is a major factor that limits the transmission capacity or transmission distance in long-distance transmission.
Meanwhile, it is known that this non-linear waveform degradation caused by large accumulation of dispersion is highly dependent upon allocation ratio in a case where the accumulated dispersion is compensated for at the sending and receiving ends, and it is also known that waveform degradation is minimized if the accumulated dispersion is compensated for half at the sending end and half at the receiving end.
Here dispersion compensation at the sending end will be referred to as "pre-dispersion compensation" and dispersion compensation at the receiving end will be referred to as "post-dispersion compensation". Allocation of dispersion compensation to the sending and receiving ends is described in detail in the specification of Japanese Patent Kokai Publication JP-A-9-46318.
It is necessary to increase the number of channels in order to raise the total transmission capacity of a transmission system. However, the usable wavelength band is limited by the amplification band of repeater amplifiers. In order to achieve high capacity, therefore, narrowing the wavelength spacing of each of the channels and multiplexing more channels within the limited wavelength band is vital.
Polarization interleave multiplexing is effective as a technique for realizing narrow channel spacing. This is a technique in which multiplexing is performed in such a manner that neighboring channels are always rendered orthogonal to each other in relation to polarized light. If the neighboring channels are orthogonal to each other with respect to polarization, separation can be achieved using polarization even in a case where there is spectrum overlap between the neighboring channels (see Optical Fiber Communication Conference 97, OFC '97 Technical Digest, paper TuJ1, 1997). To accomplish this, transmission experiments adopting the polarization interleave multiplexing in long-distance, high-capacity wavelength multiplex transmission are being conducted (see Optical Fiber Communication Conference '98, paper PD-12, 1998).
In order to carry out the polarization interleave multiplexing, it is required that the state of polarization of the signal light of all channels be fixed up to the wavelength multiplexer. In other words, it is required that all optical components up to the wavelength multiplexer have a polarization-maintaining function.
The particular problem in this case is the aforementioned pre-dispersion compensation. In most cases, dispersion compensation is carried out using optical fiber referred to as dispersion-compensating fiber. Such fiber does not possess the polarization-maintaining function.
A technique through which an optical device not originally having a polarization-maintaining function is provided with this function by using a Faraday rotator mirror is described in S. Yamashita et. al. IEEE/OSA Journal of Lightwave Technology, vol. 14, no. 3, pp. 385-390, March 1996, and in the specification of Japanese Patent Kokai Publication JP-A-8-248456.
An example in which this technique is applied to a dispersion compensation apparatus has been reported (see Optical Fiber Communication Conference 99, paper TuS5, 1999). FIG. 6 illustrates a reflecting-type dispersion compensation apparatus using the Faraday rotator mirror, which is described in the aforesaid report, and dispersion-compensating fiber.
As shown in FIG. 6, input signal light applied to an optical circulator 50 from a signal input port 51 passes through a dispersion-compensating fiber 10, is reflected by a Faraday rotator mirror 1 and passes through the dispersion-compensating fiber 10 again. Owing to the effect of the Faraday rotator mirror 1, signal light that has returned to the optical circulator 50 has orthogonal polarization to the signal light input to the signal input port 51. Polarized output light thus made orthogonal to the input signal light is delivered from a signal output port 52 of the optical circulator 50.